Ethereum Classic, the Ethereum Blockchain that is designed to be immutable, unlike the Ethereum Blockchain, which forked to bail out the DAO investors from a hack due to bad smart contract (not ethereum) code. The ethereum classic community hard-forked again, with success. The new version, codenamed diehard was released and implemented on January 13, 2017. This was not to change the blockchain, but was to fix a difficulty bomb that was implemented before Ethereum classic existed as such.
The original Ethereum before the fork had the difficulty bomb added to trigger around 2017, to make it very difficult to mine via POW; the idea being that it would move to a proof-of-stake consensus algorithm. At least 1000 ether are needed to participate. This could mean only wealthy ethereum holders would get to participate, the average joe with less than 1000 ether would find this difficult.
Ethereum classic has delayed this ‘difficulty bomb’ for a year, to give the community time to decide which algorithm to choose. There has been some negativity on Ethereum forking so many times, however it has to be noted that in the early days, Bitcoin hard-forked to fix some serious bugs, including a bug that generated many ‘fake’ Bitcoins which was resolved via hard-fork.
Ethereum is still a young platform, but with much potential. Both Ethereum and Ethereum classic are viable, but if you want immutable smart contracts with an immutable blockchain, Ethereum classic offers this more than Ethereum, as history has shown. It is unlikely however they would fork again in this scenario either way.
For now, both flavours of Ethereum can be mined via POW. It is likely that the original Ethereum will switch to proof-of-stake, and at this point you will not be able to mine it with GPUs. It is likely at this point that Ethereum Classic will see a boost in hashrate as a result.